Flow deflector fob nuc ear fuel element assemblies

ABSTRACT

10. A NUCLEAR REACTOR FUEL ASSEMBLY COMPRISING A PLURALITY LONGITUDINALLY EXTENDING PARALLEL FUEL ELEMENTS ARRANGED IN SPACED ROWS FORMING LONGITUDINAL COOLANT FLOW CHANNELS THEREBETWEEN; A GRID FORMED FROM A PLURALITY OF CROSS-LACED INTERENGAGING STRAPS, EACH OF SAID STRAPS BEING LOCATED BETWEEN SAID SPACED ROWS OF FUEL ELEMENTS; AND A PLURALITY OF DEFLECTOR CONES LOCATED IN SAID FLOW CHANNELS BETWEEN SAID FUEL ELEMENTS, SAID CONES BEING ATTACHED TO SAID GRID AT THE INTERSECTIONS OF SAID STRAPS, SAID CONES EACH BEING GENERALLY PYRAMIDAL IN SHAPE HAVING AN APEX AND A GENERALLY SQUARE BASE AND FOUR DEFLECTING SIDEWALL SURFACES, EACH OF SAID SIDEWALL SURFACES BEING ORIENTED SO AS TO FACE THE ADJACENT FUEL ELEMENT AND EACH SIDEWALL SURFACE BEING CONCAVE WITH THE CENTER OF CURVATURE OF SAID CONCAVITY GENERALLY CORRESPONDING TO THE CENTER OF SAID ADJACENT FUEL ELEMENT WHEREBY AN INTERMITTENT ANNULAR SPACE IS FORMED BETWEEN EACH FUEL ELEMENT AND THE ADJACENT DEFLECTOR CONES, THE APEX OF SAID CONES BEING LOCATED UPSTREAM AND SAID BASE BEING LOCATED DOWNSTREAM WITH RESPECT TO THE DIRECTION OF SAID COOLANT FLOW.

IE 25,362 I Liv i {iffy March J. I. uni-VIN FLOW DEFLSCTOR FOB NUC. EARFUEL ELEMENT ASSEMBLIES Original Filed Dec. 31, 1969 2 Sheets-Sheet 1j/l/l/f/l/ra? JW/A/ /V. CAM l///V 5y M Q/M i7 7 J/Q/VE 1 March 11, 1975J. N. CALVIN Re. 28,362

FLOW DEFLECTOR FOR NUCJMR FUEL ELEMENT ASSEMBLIES Original Filed Dec.31, 1969 FIGJI &J FIG.9 41

2 Sheets-Sheet 2 United States Patent Re. 28,362 Reissued Mar. 11, 197528,362 FLOW DEFLECTOR FOR NUCLEAR FUEL ELEMENT ASSEMBLIES John NormanCalvin, West Simsbury, Coun., assignor to Combustion Engineering, Inc.,Windsor, Conn. Original No. 3,663,367, dated May 16, 1972, Ser. No.889,548, Dec. 31, 1969. Application for reissue Dec. 26, 1972, Ser. No.317,977

Int. Cl. GZlc 3/30 US. Cl. 176-78 4 Claims Matter enclosed in heavybrackets 1 appears in the original patent but forms no part of thisreissue specification; matter printed in italics indicates the additionsmade by reissue.

ABSTRACT OF THE DISCLOSURE A nuclear reactor fuel element assemblycontaining a plurality of spaced parallel fuel elements is provided withinverted conical or pyramidal deflector elements in the coolant flowchannels between the fuel elements to divert the coolant flow. Thisdisrupts the coolant layer adjacent to the fuel elements, promotes themixing of coolant from various channels and raises the critical heatflux. The deflector elements are supported in position at the points ofintersection of a grid structure which may also be a support grid forthe fuel elements.

BACKGROUND OF THE INVENTION It is well known that the fuel orfissionable material for heterogeneous nuclear reactors isconventionally in the form of fuel elements or rods which are in turngrouped together in the reactors in bundles comprising fuel elementassemblies. Each reactor has a number of such fuel element assembliestherein comprising the reactor core. The liquid moderator-coolant,normally water, flows upwardly through the reactor core in the channelsbetween the fuel elements to remove heat.

One of the operating limitations on current reactors is established bythe onset of film boiling on the surfaces of the fuel elements. Thisphenomenon is commonly described qualitatively as departure fromnucleate boiling (DNB) and quantitatively in terms of the amount of heatflux existing when DNB occurs (critical heat flux or CHF). Thiscondition is affected by the fuel element spacing, the system pressure,the heat flux, the coolant enthalpy and the coolant velocity. When DNBoccurs, there is a rapid rise in the temperature of the adjacent fuelelement due to the reduced heat transfer which could result in a failureof the element. Therefore, in order to maintain a factor of safety, thereactor must be operated a certain margin below the CHF and the point atwhich DNB occurs. This margin is referred to as the thermal margin.

Nuclear reactors normally have regions in the core which have a higherneutron flux and power density than other regions. This may be caused bya number of factors, one of which is the presence of control rodchannels in the core. When the control rods are withdrawn, the controlrod channels are filled with moderator which increases the localmoderating capacity and thereby increases the power generated in theadjacent fuel. In these regions of high power density, known as hotchannels," there is a higher rate of coolant enthalpy rise than in otherchannels. It is such hot channels that set the maximum operatingconditions for the reactor and limit the amount of power that can begenerated since it is in these channels that the critical thermal marginwould be reached first.

SUMMARY OF THE INVENTION It has been found that coolant flow inclined tothe fuel elements will result in a higher value for the critical heatflux probably because such flow inhibits the formation of steam bubblesand superheated water layers which are found to exist over the fuelelement surface just prior to DNB in the presence of parallel flow. Ithas also been found that mixing vanes or flow deflectors placed in thecoolant flow channels of a reactor core will mix coolant from variouschannels and thus tend to reduce the effect of hot channels. The mixinglowers the high coolant enthalpy rise in the hot channels and tends toaverage out the enthalpy rise over the core cross section. Both effectsmean that the reactor can be operated at a higher power level and stillmaintain a safe thermal margin.

It is therefore an object of the present invention to provide novelcoolant flow deflectors in the reactor core.

Another object of the invention is to provide flow deflectors which willeffectively cause disturbance of the coolant flow adjacent the surfacesof the fuel elements as well as cause mixing of the coolant from variouschannels.

A further object is to provide flow deflectors of a novel design whichare supported in the core in a novel manner.

Briefly, the objects of the invention are accomplished by providing acoolant flow deflector in one or more of the spaces between adjacentfuel elements. More specifically, the deflectors are generally of aconical or pyramidal shape and are supported such that flow will bediverted from the centers of the flow channels up against or towards theadjacent fuel elements. Even more specifically, the flow deflectors aresupported in the intersections of a grid which may also serve as asupport grid for the fuel elements.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevation view of a fuelassembly.

FIG. 2 is a perspective view of a portion of a fuel element assemblyillustrating the present invention.

FIG. 3 is a section view through a fuel assembly looking downwardlytowards a spacer grid.

FIGS. 4 and 5 illustrate portions of grid-forming members.

FIGS. 6, 7 and 8 are side, top and bottom views, respectively, of asolid deflector.

FIGS. 9, and 11 are side, top and bottom views, respectively, of ahollow deflector.

FIG. 12 is an exploded view illustrating the assembly technique for thedeflector of FIGS. 9, 10, and 11.

FIG. 13 is an exploded view illustrating the assembly technique for amodified deflector and a fuel element spacer ring.

FIG. 14 is a cross-section view illustrating the deflector and spacer ofFIG. 13 in relation to the fuel elements.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates a fuelelement assembly 10 containing individual fuel elements 12 held in placeby end fittings 14 and 16 and spacer grids 18. The core of a nuclearreactor is formed from a plurality of such fuel assemblies and it is insuch assemblies that the coolant flow deflectors of the presentinvention are incorporated. The reactor coolant, which is normallywater, flows up through apertures in the lower end fitting 16 andupwardly parallel with the fuel elements and out through the upper endfitting 14. It is this upward coolant flow through the fuel assemblieswhich is affected by the deflectors.

The coolant flow deflectors are mounted on grid support structureslocated at intervals along the length of the fuel assemblies. Thesegrids may be for the sole purpose of supporting the deflectors as shownin FIG. 2 or they may be for the additional purpose of spacing andholding the fuel elements as shown in FIG. 3. FIG. 2 illustrates twofuel elements 12 and a deflector 20 mounted on a grid structure 22 whichis formed of members 24 and 26. The deflector 20 is generally pyramidaland deflects the coolant flow from the center of the flow channelbetween four fuel elements outwardly towards the fuel elements asindicted by the arrows. This flow deflection has two primary effects.First, the flow deflection disrupts the coolant flow conditionsimmediately adjacent the surface of the individual fuel elements. Thistends to eliminate any DNB condition. There is a gradual change fromnucleate to stable film boiling rather than a step change. The criticalheat flux is increased and it is even difficult to detect the criticalpoint due to the gradual change in boiling characteristics. Second, theflow deflection tends to cause the coolant flowing upwardly in anyparticular flow channel between the fuel elements to be mixed with thecoolant flow in adjacent and even more remote channels. This has theeffect of evening out differences in coolant temperature between variouschannels.

'FIG. 3 illustrates the invention applied to a fuel element spacer grid18, and it is a cross-section view looking downwardly on one of suchgrids. The grid has projections 28 and springs 30 formed in each gridcompartment for engaging and supporting the fuel elements 12, only oneof which is illustrated. Of course, the illustrated spacer grid is onlyone of many types to which the deflectors of the present invention couldbe applied.

The grid-forming straps 24 and 26 are illustrated in more detail inFIGS. 4 and 5. They are slotted at 32 and 34, respectively, and theseslots fit together in egg crate fashion to form the grid. The cut-outs36 are to accept the lower ends of the deflector 20 as shown in FIG. 2.

The deflectors may be of any desired tapered shape which will be definedas conical. Although the deflectors could be for example a rightcylindrical cone, the preferred conical shape is the generally pyramidalshape shown in the drawings. The pyramid deflector 20 shown in FIGS. 6,7 and 8 is of solid construction with slots 38 and 40 into which thegrid-forming members 4 and fit. The lower end 42 of the deflector neednot come to a sharp point and is truncated. The deflector has slightlyconcave side faces as shown in FIGS. 2, 3, 7 and 8 so as to make theannular space between the deflectors 20 and the fuel elements 12relatively constant as shown in FIG. 3. This is accomplished by havingthe center of the radius of curvature of the concavity correspond to thecenter of the adjacent fuel element. The deflectors are welded to thegrid-forming members to hold them in position.

FIGS. 9, and 11 illustrate a modified form of deflector 44 which is alsopyramidal in shape but which is formed of sheet metal rather than beingsolid. This deflector has a slot 46 in the lower end for accepting oneof the grid-forming members and another slot 48 in the upper end foraccepting the other grid-forming member. This deflector 44 has beenillustrated with flat side faces but they could also be concave as inthe deflector in FIGS. 6, 7 and 8. FIG. 12 illustrates the configurationof the grid-forming members 50 and 52 which are used in conjunction withthe deflectors 44. These grid-forming members have slots 54 and 56 forassembling the two members together and cut-outs 58 and 60 which conformto the contour of the deflector 44 and permit assembly. The edges of thecut-outs 58 identified as 59 fit against the inside of the deflector inthe pointed end while the edges 61 of the cut-outs 60 conform to theoutside of the deflector adjacent the base or upper end. The straightedges 62 of the cut-outs permit the grid-forming members to be insertedinto the deflector slots 46 and 48. The deflector is then trappedbetween the grid-forming members and welding or other forms ofattachment need not be depended upon to keep the deflector in place.

FIGS. 13 and 14 illustrate a further modification of the presentinvention in which the deflector and grid-forming members are designedso as to provide for the inclusion of a fuel element spacer and supportring 63. This ring fits into the enlarged cut-out portion 64 in theslots 66 in the grid-forming members 68 and 70 and act as a springmember to support the fuel elements 12 as illustrated in FIG. 14. Thegrid-forming members 68 and 70 include slots 72 and 74 for assemblingthem in egg crate" fashion. The pyramidal deflector 76 has edges 77 atthe juncture between adjacent side walls. A portion 78 of each side wallat the juncture extends outwardly parallel to and spaced from theadjacent outwardly extending portion. This forms a space between the twoparallel portions into which the grid-forming members fit. Thegrid-forming member 70 fits down into slots 80 in the upper ends of twoopposite extending portions while the grid-forming member 68 fitsupwardly into the slots 82 in the lower ends of the other two extendingportions 78. This design has the added feature that the spacer ring canbe formed of a different material than that of the grid-forming membersand deflectors. This means that this ring 63 can be made of a materialsuch as Inconel which will retain its spring characteristicssatisfactorily during reactor operation while the grid-forming membersand deflectors can be formed from a material such as Zircoloy which hasa lower neutron absorption cross section and which will tend to losesome spring when subjected to radiation.

While several embodiments of the invention have been shown anddescribed, it will be understood that such is merely illustrative andthat changes may be made without departing from the scope of theinvention as claimed.

I claim:

[1. A nuclear reactor fuel assembly comprising a plurality oflongitudinally extending parallel fuel elements arranged in a spacedarray forming longitudinal coolant flow channels therebetween, aplurality of deflector cones located in said flow channels between saidfuel elements, said deflector cones each including an apex and a baseand deflecting surfaces, said apex being located upstream and said basebeing located downstream with respect to the direction of said coolantflow, and means supporting said deflector cones in position] [2. Anuclear reactor fuel assembly as recited in claim 1 wherein saiddeflecting surfaces are oriented so as to deflect said coolant flowoutwardly in said coolant channels generally towards adjacent fuelelements] [3. A nuclear reactor fuel assembly as recited in claim 1wherein said deflector cones are each generally pyramidal in shapehaving a square base and four deflecting side wall surfaces, said sidewall surfaces being oriented so as to face four adjacent fuel elements][4. A nuclear reactor fuel assembly as recited in claim 3 wherein saidside wall surfaces are concave and wherein the center of curvature ofsaid concavity generally corresponds to the center of said adjacent fuelelement whereby an intermittent annular space is formed between eachfuel element and the adjacent deflector cones] [5. A nuclear reactorfuel assembly as recited in claim 1 wherein said means supporting saiddeflector cones comprises a grid formed from a plurality of cross-lacedinter engaging straps, said cones being attached to said grid at theintersections of said straps] [6. A nuclear reactor fuel assembly asrecited in claim 5 wherein said grid includes means for engaging andsupporting said fuel elements] 7. A nuclear reactor fuel assembly asrecited in claim [5] 10 wherein each of said deflector cones includes aslot in one end thereof extending from one corner to the opposite cornerand a slot in the opposite end thereof extending between the other twocorners and wherein a first strap extending in one direction passesthrough one slot and a strap extending perpendicularly to said firststrap passes through the other slot whereby said deflector cone istrapped between said straps.

8. A nuclear reactor fuel assembly comprising a plurality oflongitudinally extending parallel fuel elements arranged in a pluralityof intersecting rows and forming longitudinal coolant flow channelstherebetween, a grid structure formed from a plurality of cross-lacedinterengaging straps, said straps interposed between adjacent rows ofsaid fuel elements forming fuel element compartments, a plurality ofgenerally pyramidal deflector cones located in said coolant flowchannels and mounted on said grid structure at the intersection of saidstraps, said deflector cones having square bases located downstream withrespect to the direction of coolant flow and apexes located upstream,and a plurality of fuel element support rings surrounding said deflectorcones, said rings extending into said fuel element compartments so as toengage and support said fuel elements.

9. A nuclear reactor fuel assembly as recited in claim 8 wherein saiddeflector cones each have four deflecting side walls each oriented so asto face an adjacent fuel element, said deflector cones each being formedfrom sheet metal and wherein the portions of each side wall at thejuncture of said side walls extend outwardly parallel to and spaced fromthe adjacent outwardly extending side wall portion, said adjacentoutwardly extending portions terminating and joined together in a lineextending vertically from the corners of said base, slots formed in saidportions where joined together, said slots being formed only partiallythrough two opposite portions from said base end and only partiallythrough the other two opposite portions from said apex end, saidcross-laced interengaging straps containing vertical slots which areinterengaged with said slots in said portions from opposite ends of saiddeflector cone whereby said deflector cone is held between said straps,said vertical slots further including cut-outs in which said supportrings are positioned around said deflector cones.

10. A nuclear reactor fuel assembly comprising a pluralitylongitudinally extending parallel fuel elements arranged in spaced rowsforming longitudinal coolant flow channels therebetween; a grid formedfrom a plurality of cross-laced interengaging straps, each of saidstraps being located between said spaced rows of fuel elements; and aplurality of deflector cones located in said flow channels between saidfuel elements, said cones being attached to said grid at theintersections of said straps, said cones each being generally pyramidalin shape having an apex and a generally square base and four deflectingsidewall surfaces, each of said sidewall surfaces being oriented so asto face the adjacent fuel element and each sidewall surface beingconcave with the center of curvature of said concavity generallycorresponding to the center of said adjacent fuel element whereby anintermittent annular space is formed between each fuel element and theadjacent deflector cones, the apex of said cones being located upstreamand said base being located downstream with respect to the direction ofsaid coolant flow.

References Cited The following references, cited by the Examiner, are ofrecord in the patented file of this patent or the original patent.

UNITED STATES PATENTS 3,298,922 1/1967 Prince et al. 17678 3,301,7641/1967 Timbs et al 17678 3,344,855 10/1967 Clark 17678 X 3,350,27510/1967 Venier et al. 17676 X 3,356,587 12/1967 Heck 17678 3,379,6184/1968 Frisch 17678 3,439,737 4/1969 Boorman et a1. 17678 X 3,510,3975/1970 Zettervall l76 78 3,379,619 4/1968 Andrews et a1. 17678 3,393,1287/1968 Obertelli et a1 l7678 LELAND A. SEBASTIAN, Primary Examiner P. K.PAVEY, Assistant Examiner

